﻿//This is a reduced copy of .Net's concurrent queue, which is not available on all mobile platforms, and
//has therefore been added here directly.

// ==++==
//
// Copyright (c) Microsoft Corporation.  All rights reserved. 
//
// Edited by Thomas Scheller (reduced to minimum required functionality) 
//
// ==--== 
// =+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ 
//
// ConcurrentQueue.cs 
//
// <owner>[....]</owner>
//
// A lock-free, concurrent queue primitive. 
//
// =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=- 

#pragma warning disable 0420 //disable volatile fields warnings

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Threading;
//using System.Security.Permissions;
#if SILVERLIGHT
using SpinWait = XcoAppSpaces.Core.Ports.Utils.SpinWait;
#endif

namespace XcoAppSpaces.Core.Ports.Utils
{
	/// <summary>
	/// Represents a thread-safe first-in, first-out collection of objects. 
	/// </summary> 
	/// <typeparam name="T">Specifies the type of elements in the queue.</typeparam>
	/// <remarks> 
	/// All public  and protected members of <see cref="ConcurrentQueue{T}"/> are thread-safe and may be used
	/// concurrently from multiple threads.
	/// </remarks>
	//[ComVisible(false)] 
	[DebuggerDisplay("Count = {Count}")]
	//[DebuggerTypeProxy(typeof(SystemCollectionsConcurrent_ProducerConsumerCollectionDebugView<>))] 
	//[HostProtection(Synchronization = true, ExternalThreading = true)] 
	//[Serializable]
	class ConcurrentQueue<T> //: IProducerConsumerCollection<t> 
    {
		//fields of ConcurrentQueue
		private volatile Segment m_head; 
		private volatile Segment m_tail; 
		private const int SEGMENT_SIZE = 32;

		/// <summary> 
		/// Initializes a new instance of the <see cref="ConcurrentQueue{T}"> class.
		/// </see></summary> 
		public ConcurrentQueue()
		{
			Clear();
		}

		/// <summary>
		/// Clears the queue by re-initializing it.
		/// </summary>
		public void Clear()
		{
			m_head = m_tail = new Segment(0); 
		}

		/// <summary> 
		/// Gets a value that indicates whether the <see cref="ConcurrentQueue{T}"> is empty.
		/// </see></summary> 
		/// <value>true if the <see cref="ConcurrentQueue{T}"> is empty; otherwise, false.</see></value>
		/// <remarks>
		/// For determining whether the collection contains any items, use of this property is recommended
		/// rather than retrieving the number of items from the <see cref="Count"/> property and comparing it 
		/// to 0.  However, as this collection is intended to be accessed concurrently, it may be the case
		/// that another thread will modify the collection after <see cref="IsEmpty"> returns, thus invalidating 
		/// the result. 
		/// </see></remarks>
		public bool IsEmpty 
		{
            get
            {
                Segment head = m_head; 
                if (!head.IsEmpty)
                    //fast route 1: 
                    //if current head is not empty, then queue is not empty 
                    return false;

                else if (head.Next == null) 
                    //fast route 2:
                    //if current head is empty and it's the last segment
                    //then queue is empty
                    return true; 

                else
                //slow route: 
                //current head is empty and it is NOT the last segment, 
                //it means another thread is growing new segment
                { 
                    SpinWait spin = new SpinWait();
                    while (head.IsEmpty)
                    {
                        if (head.Next == null) 
                            return true;

                        spin.SpinOnce(); 
                        head = m_head;
                    } 
                    return false;
                }
            }
        } 

        /// <summary>
        /// Store the position of the current head and tail positions.
        /// </summary>
        /// <param name="head">return the head segment</param>
		/// <param name="tail">return the tail segment</param>
		/// <param name="headLow">return the head offset</param>
		/// <param name="tailHigh">return the tail offset</param>
        private void GetHeadTailPositions(out Segment head, out Segment tail, out int headLow, out int tailHigh) 
        {
            head = m_head;
            tail = m_tail;
            headLow = head.Low; 
            tailHigh = tail.High;

            SpinWait spin = new SpinWait(); 

            //we loop until the observed values are stable and sensible.
            //This ensures that any update order by other methods can be tolerated. 
			while (
                //if head and tail changed, retry
                head != m_head || tail != m_tail
                //if low and high pointers, retry 
                || headLow != head.Low || tailHigh != tail.High
                //if head jumps ahead of tail because of concurrent grow and dequeue, retry 
                || head.m_index > tail.m_index) 
            {
                spin.SpinOnce(); 
                head = m_head;
                tail = m_tail;
                headLow = head.Low;
                tailHigh = tail.High; 
            }
        } 

  
        /// <summary> 
        /// Gets the number of elements contained in the <see cref="ConcurrentQueue{T}">.
        /// </see></summary>
        /// <value>The number of elements contained in the <see cref="ConcurrentQueue{T}">.</see></value>
        /// <remarks> 
        /// For determining whether the collection contains any items, use of the <see cref="IsEmpty"/>
        /// property is recommended rather than retrieving the number of items from the <see cref="Count"> 
        /// property and comparing it to 0. 
        /// </see></remarks>
        public int Count 
        {
            get
            {
                //store head and tail positions in buffer, 
                Segment head, tail;
                int headLow, tailHigh; 
                GetHeadTailPositions(out head, out tail, out headLow, out tailHigh); 

                if (head == tail) 
                {
                    return tailHigh - headLow + 1;
                }

                //head segment
                int count = SEGMENT_SIZE - headLow; 

                //middle segment(s), if any, are full.
                //We don't deal with overflow to be consistent with the behavior of generic types in CLR. 
                count += SEGMENT_SIZE * ((int)(tail.m_index - head.m_index - 1));

                //tail segment
                count += tailHigh + 1; 

                return count; 
            } 
        }
 

		/// <summary>
        /// Adds an object to the end of the <see cref="ConcurrentQueue{T}">.
        /// </see></summary>
        /// <param name="item">The object to add to the end of the <see cref="ConcurrentQueue{T}"/>. The value can be a null reference 
		/// (Nothing in Visual Basic) for reference types.</param>
        public void Enqueue(T item) 
        {
            SpinWait spin = new SpinWait(); 
            while (true)
            {
                Segment tail = m_tail;
                if (tail.TryAppend(item, ref m_tail)) 
                    return;
                spin.SpinOnce(); 
            } 
        }

        /// <summary>
        /// Attempts to remove and return the object at the beginning of the <see cref="ConcurrentQueue{T}">. 
        /// </see></summary>
        /// <param name="result"> 
        /// When this method returns, if the operation was successful, <paramref name="result"/> contains the 
		/// object removed. If no object was available to be removed, the value is unspecified.</param>
        ///  
        /// <returns>true if an element was removed and returned from the beggining of the <see cref="ConcurrentQueue{T}">
        /// succesfully; otherwise, false.</see></returns>
        public bool TryDequeue(out T result)
        {
            while (!IsEmpty) 
            { 
                Segment head = m_head;
                if (head.TryRemove(out result, ref m_head))
                    return true;
                //since method IsEmpty spins, we don't need to spin in the while loop
            }
            result = default(T); 
            return false;
        } 

  

        /// <summary>
        /// Attempts to return an object from the beginning of the <see cref="ConcurrentQueue{T}"> 
        /// without removing it.
        /// </see></summary>
        /// <param name="result">When this method returns, <paramref name="result"/> contains an object from
        /// the beginning of the <see cref="T:System.Collections.Concurrent.ConccurrentQueue{T}"/> or an 
		/// unspecified value if the operation failed.</param>
        /// <returns>true if and object was returned successfully; otherwise, false.</returns> 
        public bool TryPeek(out T result) 
        {
            while (!IsEmpty)
            {
                Segment head = m_head;
                if (head.TryPeek(out result))
                    return true; 
                //since method IsEmpty spins, we don't need to spin in the while loop
            } 

            result = default(T); 
            return false;
        } 


        /// <summary>
        /// private class for ConcurrentQueue. 
        /// a queue is a linked list of small arrays, each node is called a segment.
        /// A segment contains an array, a pointer to the next segment, and m_low, m_high indices recording 
        /// the first and last valid elements of the array. 
        /// </summary>
        private class Segment 
        {
            //we define two volatile arrays: m_array and m_state. Note that the accesses to the array items
            //do not get volatile treatment. But we don't need to worry about loading adjacent elements or
            //store/load on adjacent elements would suffer reordering. 
            // - Two stores:  these are at risk, but CLRv2 memory model guarantees store-release hence we are safe.
            // - Two loads: because one item from two volatile arrays are accessed, the loads of the array references 
            //          are sufficient to prevent reordering of the loads of the elements. 
            internal volatile T[] m_array;

            //Each array entry has 2 possible states
            // 0 -- initial
            // 1 -- contains value: it may be dequeued, but value is still accessible
            // todo: change this int array to bitmap 
            private volatile int[] m_state;

            //pointer to the next segment. null if the current segment is the last segment 
            private volatile Segment m_next;

            //We use this zero based index to track how many segments have been created for the queue, and
            //to compute how many active segments are there currently.
            // * The number of currently active segments is : m_tail.m_index - m_head.m_index + 1;
            // * m_index is incremented with every Segment.Grow operation. We use Int64 type, and we can safely 
            //   assume that it never overflows. To overflow, we need to do 2^63 increments, even at a rate of 4
            //   billion (2^32) increments per second, it takes 2^31 seconds, which is about 64 years. 
            internal readonly long m_index; 

            //indices of where the first and last valid values 
            // - m_low points to the position of the next element to pop from this segment, range [0, infinity)
            //      m_low >= SEGMENT_SIZE implies the segment is disposable
            // - m_high points to the position of the latest pushed element, range [-1, infinity)
            //      m_high == -1 implies the segment is new and empty 
            //      m_high >= SEGMENT_SIZE-1 means this segment is ready to grow.
            //        and the thread who sets m_high to SEGMENT_SIZE-1 is responsible to grow the segment 
            // - Math.Min(m_low, SEGMENT_SIZE) > Math.Min(m_high, SEGMENT_SIZE-1) implies segment is empty 
            // - initially m_low =0 and m_high=-1;
            private volatile int m_low; 
            private volatile int m_high;

            /// <summary>
            /// Create and initialize a segment with the specified index. 
            /// </summary>
            internal Segment(long index) 
            { 
                m_array = new T[SEGMENT_SIZE];
                m_state = new int[SEGMENT_SIZE]; //all initialized to 0 
                m_high = -1;
                //Contract.Assert(index >= 0);
                m_index = index;
            } 

            /// <summary> 
            /// return the next segment 
            /// </summary>
            internal Segment Next 
            {
                get { return m_next; }
            }

            /// <summary> 
            /// return true if the current segment is empty (doesn't have any element available to dequeue, 
            /// false otherwise
            /// </summary> 
            internal bool IsEmpty
            {
                get { return (Low > High); }
            } 


			///// <summary> 
			///// Add an element to the tail of the current segment 
			///// exclusively called by ConcurrentQueue.InitializedFromCollection
			///// InitializeFromCollection is responsible to guaratee that there is no index overflow, 
			///// and there is no contention
			///// </summary>
			///// <param name="value">
			//internal void UnsafeAdd(T value) 
			//{
			//	Contract.Assert(m_high < SEGMENT_SIZE - 1); 
			//	m_high++; 
			//	m_array[m_high] = value;
			//	m_state[m_high] = 1; 
			//}
 

			///// <summary>
			///// Create a new segment and append to the current one 
			///// Does not update the m_tail pointer
			///// exclusively called by ConcurrentQueue.InitializedFromCollection 
			///// InitializeFromCollection is responsible to guaratee that there is no index overflow, 
			///// and there is no contention
			///// </summary> 
			///// <returns>the reference to the new Segment</returns>
			//internal Segment UnsafeGrow()
			//{
			//	Contract.Assert(m_high >= SEGMENT_SIZE - 1); 
			//	Segment newSegment = new Segment(m_index + 1); //m_index is Int64, we don't need to worry about overflow
			//	m_next = newSegment; 
			//	return newSegment; 
			//}

  
            /// <summary>
            /// Create a new segment and append to the current one
            /// Update the m_tail pointer
            /// This method is called when there is no contention 
            /// </summary>
            internal void Grow(ref Segment tail) 
            { 
                //no CAS is needed, since there is no contention (other threads are blocked, busy waiting)
                Segment newSegment = new Segment(m_index + 1);  //m_index is Int64, we don't need to worry about overflow 
                m_next = newSegment;
                //Contract.Assert(tail == this);
                tail = m_next;
            } 


            /// <summary> 
            /// Try to append an element at the end of this segment.
            /// </summary> 
			/// <param name="value">the element to append</param>
			/// <param name="tail">The tail.</param>
            /// <returns>true if the element is appended, false if the current segment is full</returns>
            /// <remarks>if appending the specified element succeeds, and after which the segment is full, 
            /// then grow the segment</remarks>
            internal bool TryAppend(T value, ref Segment tail)
            { 
                //quickly check if m_high is already over the boundary, if so, bail out
                if (m_high >= SEGMENT_SIZE - 1)
                {
                    return false;
                }

                //Now we will use a CAS to increment m_high, and store the result in newhigh.
                //Depending on how many free spots left in this segment and how many threads are doing this Increment 
                //at this time, the returning "newhigh" can be 
                // 1) < SEGMENT_SIZE - 1 : we took a spot in this segment, and not the last one, just insert the value
                // 2) == SEGMENT_SIZE - 1 : we took the last spot, insert the value AND grow the segment 
                // 3) > SEGMENT_SIZE - 1 : we failed to reserve a spot in this segment, we return false to
                //    Queue.Enqueue method, telling it to try again in the next segment.

                int newhigh = SEGMENT_SIZE; //initial value set to be over the boundary 

                //We need do Interlocked.Increment and value/state update in a finally block to ensure that they run 
                //without interuption. This is to prevent anything from happening between them, and another dequeue 
                //thread maybe spinning forever to wait for m_state[] to be 1;
                try
                { }
                finally
                {
                    newhigh = Interlocked.Increment(ref m_high); 
                    if (newhigh <= SEGMENT_SIZE - 1)
                    { 
                        m_array[newhigh] = value; 
                        m_state[newhigh] = 1;
                    } 

                    //if this thread takes up the last slot in the segment, then this thread is responsible
                    //to grow a new segment. Calling Grow must be in the finally block too for reliability reason:
                    //if thread abort during Grow, other threads will be left busy spinning forever. 
                    if (newhigh == SEGMENT_SIZE - 1)
                    { 
                        Grow(ref tail); 
                    }
                } 

                //if newhigh <= SEGMENT_SIZE-1, it means the current thread successfully takes up a spot
                return newhigh <= SEGMENT_SIZE - 1;
            } 


            /// <summary> 
            /// try to remove an element from the head of current segment
            /// </summary> 
			/// <param name="result">The result.</param>
			/// <param name="head">The head.</param>
            /// <returns>return false only if the current segment is empty</returns>
            internal bool TryRemove(out T result, ref Segment head) 
            {
                SpinWait spin = new SpinWait(); 
                int lowLocal = Low, highLocal = High; 
                while (lowLocal <= highLocal)
                { 
                    //try to update m_low
                    if (Interlocked.CompareExchange(ref m_low, lowLocal + 1, lowLocal) == lowLocal)
                    {
                        //if the specified value is not available (this spot is taken by a push operation, 
                        // but the value is not written into yet), then spin
                        SpinWait spinLocal = new SpinWait(); 
                        while (m_state[lowLocal] == 0) 
                        {
                            spinLocal.SpinOnce(); 
                        }
                        result = m_array[lowLocal];

                        //if the current thread sets m_low to SEGMENT_SIZE, which means the current segment becomes 
                        //disposable, then this thread is responsible to dispose this segment, and reset m_head
                        if (lowLocal + 1 >= SEGMENT_SIZE) 
                        { 
                            //  Invariant: we only dispose the current m_head, not any other segment
                            //  In usual situation, disposing a segment is simply seting m_head to m_head.m_next 
                            //  But there is one special case, where m_head and m_tail points to the same and ONLY
                            //segment of the queue: Another thread A is doing Enqueue and finds that it needs to grow,
                            //while the *current* thread is doing *this* Dequeue operation, and finds that it needs to
                            //dispose the current (and ONLY) segment. Then we need to wait till thread A finishes its 
                            //Grow operation, this is the reason of having the following while loop
                            spinLocal = new SpinWait(); 
                            while (m_next == null)
                            {
                                spinLocal.SpinOnce();
                            }
                            //Contract.Assert(head == this);
                            head = m_next;
                        } 

                        return true; 
                    } 
                    else
                    { 
                        //CAS failed due to contention: spin briefly and retry
                        spin.SpinOnce();
                        lowLocal = Low; highLocal = High;
                    } 
                }//end of while
                result = default(T); 
                return false; 
            }

  
            /// <summary>
            /// try to peek the current segment
            /// </summary>
            /// <param name="result">holds the return value of the element at the head position, 
			/// value set to default(T) if there is no such an element</param>
            /// <returns>true if there are elements in the current segment, false otherwise</returns> 
            internal bool TryPeek(out T result) 
            {
                result = default(T); 
                int lowLocal = Low;
                if (lowLocal > High)
                    return false;
                SpinWait spin = new SpinWait(); 
                while (m_state[lowLocal] == 0)
                { 
                    spin.SpinOnce(); 
                }
                result = m_array[lowLocal];
                return true;
            }


            /// <summary> 
            /// return the position of the head of the current segment
            /// </summary> 
            internal int Low
            {
                get
                { 
                    return Math.Min(m_low, SEGMENT_SIZE);
                } 
            } 

            /// <summary> 
			/// return the logical position of the tail of the current segment
			/// </summary>
			internal int High
            { 
                get
                {
                    //if m_high > SEGMENT_SIZE, it means it's out of range, we should return 
                    //SEGMENT_SIZE-1 as the logical position
                    return Math.Min(m_high, SEGMENT_SIZE - 1); 
                }
            } 
        } 
    }//end of class Segment
}
#pragma warning restore 0420